Saint Thomas Aquinas credits with introducing the principle of doctrine of double effect in his discussion of the permissibility of self-defense in the book Summa Theologica.

We know that the major focus of ethics is on Human Actions. In simple terms, those actions which are deliberately and consciously performed by humans through their intellect and will are known as Human Actions. However, Saint Thomas Aquinas described that usually the acts are of two types viz. Human Acts & Act of Humans. Among them there are some Human actions which have both good and bad consequences simultaneously. Such acts are known as acts with double effects.

What is doctrine of double effect?

Doctrine of double effect: According to the principle of double effect, sometimes it is permissible to cause a harm as a side effect (or “double effect”) of bringing about a good result even though it would not be permissible to cause such a harm as a means to bringing about the same good end.


1.     The action must be inherently good.

2.     The good should not flow from the bad rather both should flow from the Action itself.

3.     Comparatively the proportion of good should be larger than the proportion of bad.

 Utility of Doctrine of Double Effect: The doctrine (or principle) of double effect is often invoked to explain the permissibility of an action that causes a serious harm, such as the death of a human being, as a side effect of promoting some good end.

Examples/ Applications of Doctrine of Double Effect:

  1. Terror Bombing Vs Tactical bombing: The terror bomber aims to bring about civilian deaths in order to weaken the resolve of the enemy: when his bombs kill civilians this is a consequence that he intends. The tactical bomber aims at military targets while foreseeing that bombing such targets will cause civilian deaths. When his bombs kill civilians this is a foreseen but unintended consequence of his actions. Even if it is equally certain that the two bombers will cause the same number of civilian deaths, terror bombing is impermissible while tactical bombing is permissible.
  2. Instance of abortion by a doctor: A doctor who believed that abortion was wrong, even in order to save the mother’s life, and might nevertheless consistently believe that it would be permissible to perform a hysterectomy on a pregnant woman with cancer. In carrying out the hysterectomy, the doctor would aim to save the woman’s life while merely foreseeing the death of the fetus. Performing an abortion, by contrast, would involve intending to kill the fetus as a means to saving the mother.

Although there are some criticisms about the doctrine, it serves as a best guide to handle certain diabolical situations or ethical dilemmas like Euthanasia, Medical termination of pregnancy etc.